Polyolefin films which are rendered more permeable to water vapor using filler loading and orientation are known.
Such films or film composites are said to be more breathable, that is to have improved, increased permeability to water vapors, while maintaining a resistance to liquid strikethrough (defined herein). Uses of such films or film composites include on a diaper the permeability of which may permit the passage of moisture vapor and air, while substantially preventing the passage of liquid. The advantages of such a film used in a diaper are that after the wearer voids, the liquid is generally retained, while much of the liquid vapor can escape decreasing the "wet feeling", and lowering the possibility of uncomfortable diaper rash.
Interdigitating grooved rollers have been used to orient either certain films or exclusively nonwoven laminates. Use of such rollers to orient (i.e., stretch) a film or nonwoven substrate is typically referred to as a ring-rolling process. To increase the water vapor transmission rate ("WVTR") of a film stretched by a ring-rolling process, it has been customary to increase either the filler loading in the formulation or the depth of engagement of the interdigitating grooves. However, both of these processing options have technical limitations on their ability to increase the WVTR of a film. Also, each option can potentially have negative effects on the physical properties of the stretched film, if a specific film's tolerance for filler loading and/or groove engagement depth is exceeded. Accordingly, there is a need for alternative means for increasing the WVTR, without negatively effecting a film's physical properties.
Also, it is desired for many applications of breathable films, such as disposable diapers, adult incontinent products, and feminine hygiene devices, to produce a film appearance that can provide the manufacturer and consumers of such products visual evidence of those products made of breathable films versus those made from non-breathable films.
U.S. Pat. No. 4,472,328, assigned to Mitsubishi Chemical Industries, Ltd., suggests a breathable polyolefin film prepared from a polyolefin/filler composition having from 20 percent to 80 percent by weight of a filler such as a surface treated calcium carbonate. A liquid or waxy hydrocarbon polymer elastomer such as a hydroxyl-terminated liquid polybutadiene was purported to produce a precursor film that could be mono-axially or biaxially stretched to make a film breathable. The breathable film described by Mitsubishi is also described in Great Britain Patent No.2,115,702, assigned to Kao Corporation. The Kao patent fuirther describes a disposable diaper prepared with a breathable film as disclosed by the Mitsubishi patent. The breathable film is used as a backing for the diaper to contain liquid.
U.S Pat. No. 4,350,655 by W. H. Hoge, assigned to Biax Fiber Film, describes a porous polyolefm synthetic paper film containing at least 50 percent by weight of a coated inorganic filler. The film composition was comprised of from 50 to 70 weight percent of an inorganic filler material coated with a silicon or titanium fatty acid ester. To produce such a film product, Hoge teaches to cool the disclosed film substrate down into a temperature range of 10.degree. C. to 70.degree. C. (i.e., 50.degree. F. to 158.degree. F. respectively) prior to stretching the film. Hoge refers to such a process as a "cold stretching " the film. Hoge also indicates that such cold stretching helps develop the desired void volume and surface ruptures per unit area so that weight percent resin content of the final product ranges from 0.18 to about 0.32 g/cm.sup.3. Moreover, the precursor film is formed without the addition of an elastomer and contains an inorganic filler surface coated with either a Si or Ti fatty acid ester. Some of the resulting films were stated to be both vapor and liquid permeable, however, at least one film (Example 3) was stated to be permeable to air.
U.S. Pat. No. 4,777,073 (Sheth) suggests a breathable film produced by stretching of a precursor film prepared from a polyolefin/filler composition. Sheth suggests that the permeability and strength, especially tear strength are improved by melt embossing the precursor film with a patterned melt embossing roller and stretching the film to impart a pattern of different film thickness having greater permeability within the areas of reduced thickness compared to the areas of greater thickness.
Most of these techniques require that a film or film composite be rendered breathable, regardless of the technique but generally through tentering (for transverse direction or TD orientation, and differential speeds of two rollers for machine direction or MD orientation), in a separate operation, prior to final construction of the end-use article, for instance the diaper, leading to expensive double processing or more expensive transport of the film rendered less dense by the tentering operation.
Among the most serious limitations, is the extreme difficulty in producing a cost effective lamination between polypropylene nonwoven materials and polyethylene breathable films. Traditional glue, hotmelt, or meltblown adhesive techniques can be used, but require the additional cost and process complexity of the gluing system and the adhesive. The preferred method of heat lamination was generally not reliable because the difference in melting points of the polypropylene nonwoven (.about.161.degree. C.) and the polyethylene film (.about.125.degree. C.). To achieve an adequate lamination bond strength between the two materials, pin holes or damage to the breathable film at the film/nonwoven bond site resulted.
Previous polypropylene breathable films, while having lamination advantages over polyethylene films, have been deficient in a number of other performance categories. Film oriented by traditional Machine Direction Orientation, Transverse Direction Orientation, or Biaxial Orientation (all known in the art) have had very low tear and impact strength.
For those product applications which do not laminate the breathable film directly to a nonwoven, or which by nature of the product a hot melt type adhesive gluing system is desirable (such as a breathable film diaper backsheet), polypropylene breathable film will be more resistant to glue burn through of the film. Thus, the use of a polypropylene breathable film helps to achieve product integrity. Also, the use of higher temperature glues, as well as a lower quantity of glue is required for adequate product bond strength.
Accordingly, there is need for an improved ring-rolling process and/or polymer film composition that can increase the WVTR of a film, without significantly diminishing the film's physical properties. There is also a commercial need for a polypropylene microporous breathable film with high tear and impact strengths well as a soft feel. Also, there is need for films produced by such an improved process to be readily distinguishable as breathable films.